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Alimentary tract
• Continuous provision– Water – Electrolytes– Nutrients
• Achieved by– Movement of food– Digestion
• Mechanical and chemical
– Absorption– Transport
Smooth muscles within the GI tract
• Layers– Longitudinal
• Length-wise
– Circular– Formation of syncitium
• Each fiber within respective layer– Connected via gap junctions
• Ion movement
Contraction of GI smooth muscles
• Continual, slow intrinsic electrical activity– Slow waves
• Not action potentials– Too low
• Generated by the interaction of interstitial cells of Cajal
– Periodic openings of channels
• Do not usually cause muscle contraction
Contraction of GI smooth muscles
• Continual, slow intrinsic electrical activity– Spike potentials
• Action potentials• Generated when the
resting potential goes over -40 mV
– Greater the rise in resting potential, greater the frequency
– Lasts longer than normal action potential (10-20 mSec)
• Generated by the movement of calcium ions
– Slower channels
• Changes in resting potentials– Depolarization
• Stretching of muscle• Acetylcholine• Stimulation of parasympathetic nerves• GI hormones
– Hyperpolarization• Epinephrine and norepinephrine• Stimulation of sympathetic nerves
• Role of calcium ions– Entrance to cells
• Slow waves– No muscle contraction
• Spike potentials
• Tonic– Continuous but not associated with slow waves
• Continuous repetitive spike potentials• Hormones and other factors• Continuous entry of calcium ions
– Not associated with changes in membrane potential
• Myenteric plexus– Mostly linear chain
• Extends entire length of the GI tract
– Controls muscle activity along the length of the GI tract
• Tonic contraction/tone of the wall
– Intensity
– Rhythm (slight)
• Myenteric plexus– Movement of peristalic
wave• Increased conduction
velocity of excitatory wave
– Inhibitory neurons• Secretion of inhibitory
peptide• Inhibition of sphincters
– Inhibits food movement
Role of ANS
• Parasympathetic– Cranial
• Vagus• Esophagus, stomach,
and pancreas
– Sacral• Large intestine and
anus• Defecation reflex
– Excitation• Increased activity
• Sympathetic– T5 and L2 of spinal
cord– Celiac and mesenteric
ganglia• Essentially innervates
entire GI tract
– Excitation• Inhibition of activity
– Smooth muscle– Neurons of enteric
nervous system
T5
L2
Afferent sensory nerve fibers
• Activation– Irritation of mucosa– Distention– Chemicals
• Inhibition or activation• Transmission of
information to the CNS– Afferent vagus nerves
(80 %)
Role of enteric nervous system
• Generation of reflexes– Integrated within the
enteric nervous system
• Local reflex
– Loop between the prevertebral sympathetic ganglia and GI tract
• Signals from lower portion of the GI tract to regulate activity of the upper GI tract or vise versa
• Loop between the spinal cord/brain stem and the GI tract– Vagus nerves from the
stomach to the brainstem
– Pain reflex (inhibitory)– Defecation reflex
Movement within the GI tract
• Propulsive movement– Peristalsis
• Generated in response to GI tract distension• Requires active myenteric plexus
– Formation of the contractile rings– Receptive relaxation
• Polarized movement– Move in one direction
• Mixing movement– Inhibition of peristalisis forward movement
• Sphincter• Churning of the content within the segment
– Local intermittent constrictive contractions
Splanchnic circulation
• Flow of blood– Afferent flow
• The GI tract • Pancreas• Spleen
– Enters liver via the portal vein
• Flow through liver sinusoids
– Exits liver via hepatic veins
• Vena cava
• Absorption of nutrients– Water soluble molecules
• 75 % temporally stored in liver
– Fats• Intestinal lymphatics• Enters circulation via thoracic duct
• Arterial supply to the GI tract– Mesenteric arteries (superior and inferior)
• Intestines
– Celiac artery• Stomach
• Branches of arteries– Muscle bundles– Intestinal villi– Submucosal vessels
• Rate of flow– Proportional to activity levels
• Active absorption increases flow by max. 8 X
– Increased flow• Vasodilators• Decreased tissue oxygen concentrations
• Counter-current exchange of oxygen– Diffusion of oxygen
from arterioles to venules without going through circulation
• Bypassed oxygen is not available for tissue metabolism